Synchronized cardiac impulses emerge from multi-scale, heterogeneous local calcium signals within and among cells of heart pacemaker tissue
AbstractThe current functional paradigm of sinoatrial node (SAN), the heart's natural pacemaker, postulates the entrainment of full-scale action potentials (APs) at one common frequency and does not involve any subcellular events or subthreshold signals. SAN cells studied in isolation, however, generate subcellular, spontaneous, rhythmic, local Ca2+ releases (LCRs) and, while each LCR is a small, subthreshold signal, the emergent LCR ensemble signal critically contributes to generation of
... generation of spontaneous APs and accounts for the heterogeneity of AP firing rates of these cells.We hypothesized that cells embedded in SAN tissue also generate heterogeneous subthreshold LCR signals of different rates and amplitudes and that self-organization of the LCRs within and among cells critically contributes to the ignition of APs that emerge from the SAN to excite the heart. To this end we combined immunolabeling with a novel technique to image microscopic Ca2+ dynamics, including both LCRs and AP-induced Ca2+ transients (APCTs) within and among individual pacemaker cells across the entire mouse SAN.Immunolabeling identified a meshwork of HCN4(+)/CX43(-) non-striated cells of different morphologies that extended along the crista terminalis from the superior vena cava to the inferior vena cava, becoming intertwined with a network of striated cells, rich in F-actin and CX43, but devoid of HCN4 immunolabeling. The earliest APCTs within a given pacemaker cycle occurred within a small area of HCN4 meshwork below the superior vena cava, just medial to the crista terminalis. Although subsequent APCT appearance throughout the SAN at low magnification resembled a continuous propagation pattern, higher magnification imaging revealed that APCT appearance at different sites within the SAN was patchy and discontinuous. Both LCR dynamics and APCTs were markedly heterogeneous in amplitudes and frequencies among SAN cells: in some cells, APCT occurrence was preceded by diastolic LCRs occurring in the same cell; other cells did not generate spontaneous APCTs, but had subthreshold LCRs only, which preceded APCT generation by adjacent cells; APCTs of various frequencies were generated steadily or in bursts.In summary, we discovered a novel, microscopic Ca2+ signalling paradigm of SAN operation that could not be detected with low-resolution, macroscopic tissue methods employed in prior studies: synchronized macroscopic signals within the SA node, including full-scale APs, emerge from heterogeneous microscopic subthreshold Ca2+ signals. This multiscale, complex process of impulse generation within the SAN resembles the emergence of organized signals from heterogeneous local signals generated within clusters of neurons within neuronal networks.